The invention relates to a process and a device that make it possible to produce a hydrogen-rich gas from a gas that contains H2S.
The technological background is illustrated by U.S. Pat. Nos. 4,481,181, 4,461,755, 4,302,434, FR-A-1 461 303 and FR-A-2 238 668.
H2S is a highly toxic compound that is present in many natural gases. It is also found in refinery gases, where it is generally obtained from decomposition reactions of sulfur-containing organic compounds that are naturally present in crude oils. The H2S is produced in particular in large amounts during hydrodesulfurization operations. These operations make it possible to lower the sulfur content of petroleum fractions by a treatment with hydrogen in the presence of suitable catalysts.
It is conventional both in a refinery as well as in natural gas production to extract the H2S that is present in the gases by a scrubbing with a suitable solvent, for example an amine-based solution. This solvent is then regenerated, generally by heating, which then produces a so-called acid gas that is rich in H2S. In a refinery, such an acid gas usually contains more than 90% H2S with minor contents of CO2, steam and hydrocarbons (methane and other heavier hydrocarbons).
Taking into account the toxicity of the H2S, this gas cannot be emitted into the atmosphere. Its combustion would generate considerable amounts of SO2 and SO3, polluting compounds of which it is also sought to reduce the releases into the atmosphere. The most generally used method for treating these acid gases consists in admitting them into a so-called Claus unit that makes it possible to convert H2S into elementary sulfur, a non-polluting compound that can be easily transported and marketed, for example for the production of sulfuric acid.
The Claus units comprise a thermal stage in which about one third of the H2S that is present in SO2 is oxidized by combustion with air (reaction (1)), and even in some cases with oxygen-enriched air. During this partial combustion, a portion of the SO2 that is formed reacts with H2S to form elementary sulfur according to Claus reaction (2):
H2S+3/2 O2xe2x86x92SO2+H2Oxe2x80x83xe2x80x83(1)
2 H2S+SO2xe2x86x923/x Sx+2 H2Oxe2x80x83xe2x80x83(2)
3 H2S+3/2 O2xe2x86x923/x Sx+3 H2Oxe2x80x83xe2x80x83(3)
The hot gas that is thus obtained is then cooled in a steam boiler to a temperature that allows the condensation of elementary sulfur. After the condensed elementary sulfur is separated, the gas that contains the residual SO2 and H2S is sent to one or more (generally 2 to 3) catalytic stages where the Claus reaction is continued. Each catalytic stage consists of a stage for reheating the gas followed by a reaction stage in the presence of a suitable catalyst and finally a stage for cooling the gas for separating by condensation the elementary sulfur that is formed in the reactor.
The Claus process that is usually used makes it possible to convert into elementary sulfur only about 95 to 97% of the H2S that is treated. To limit the releases of sulfur-containing gases into the atmosphere, the gas that exits from the Claus process is usually treated in a so-called Claus tail gas treatment unit. There is a large variety of units of this type, generally complex and almost as costly in investment as the Claus unit itself.
It is seen therefore that the conversion of H2S into elementary sulfur by this method, although effective, remains a complex and costly operation.
In addition, the overall material balance of the operation amounts to forming one mol of elementary sulfur and one mol of water (reaction of balance (3)) for each H2S mol.
The decomposition of H2S into elementary sulfur and hydrogen (reaction (4)) would allow not only H2S to be converted into elementary sulfur but also hydrogen, gas with high added value, to be produced, which is also in high demand for various refining operations of crude oils (hydrotreatments).
H2Sxe2x86x921/x Sx+H2xe2x80x83xe2x80x83(4)
This is why numerous authors have attempted to develop processes that make it possible to carry out in an economical manner such a decomposition. An examination of the various methods that are explored can be found in, for example, xe2x80x9cH2S A Potential Source of Hydrogenxe2x80x9d that was published by Sulphur No. 244, May-June 1996, pp. 37-47. To this day, however, it seems that no process is the subject of a large-scale industrial implementation, generally due to the inadequate conversion ratios or high operating costs.
The decomposition of H2S by an electrolytic method, such as the one developed by the Idemistsu Kosan Company, makes it possible to obtain an almost total conversion of H2S into elementary sulfur and hydrogen, but the use of electricity as the sole energy source leads to a high operating cost.
It is also the use of electricity as the sole energy source that penalizes the process by plasmalysis developed by the Kurchatov Russian Institute.
The xe2x80x9cHysulfxe2x80x9d process developed by the Marathon Oil Company makes it possible to obtain hydrogen by using only thermal energy, owing to the use of organic compounds that are hydrogenated by H2S in a first step, then dehydrogenated by catalytic reforming in a second step. It is generally admitted, however, that during such reduction-oxidation cycles, the organic compounds tend to deteriorate and that consequently the processes that are based on this type of reaction most often also lead to relatively high operating costs because of the supply of chemical products that are necessary for compensating the secondary degradation reactions.
The thermal cracking method developed by the Alberta Sulphur Research Laboratory (ASRL) also makes it possible to use only thermal energy for decomposing H2S, whereby this energy is supplied directly by the flame of a thermal stage of the Claus process. The outputs of conversions that are obtained are relatively low (limited to about 35%), however, primarily for two reasons:
the theoretical limitations due to the thermodynamic equilibrium between H2S, H2 and elementary sulfur; the decomposition of H2S that becomes important only at very high temperature (1100xc2x0 C. and beyond),
the difficulty in cooling very quickly the high-temperature mixture that contains hydrogen. Actually, during the cooling, the hydrogen and the elementary sulfur that are formed at high temperature have a tendency to recombine very quickly into H2S.
This last phenomenon is particularly important if it is desired to obtain a high hydrogen yield by thermal method, which therefore requires the use of very high temperatures. This phenomenon was studied in particular inside the furnaces of the Claus units (see, for example, xe2x80x9cWhat Happens to Hydrogen in a Claus Plant,xe2x80x9d published by Sulphur No. 214, May-June 1991, pp. 53-60). It is seen that with a dwell time, such that it is typically found in the boilers that are downstream from the Claus units, of between 0.5 and 1.5 seconds, the major portion of the hydrogen that is formed in a Claus furnace is recombined into H2S. Even with a time of cooling to 600xc2x0 C. of 50 ms, only 80% of the hydrogen that is formed at equilibrium at 1300xc2x0 C. in a Claus furnace would be recovered. In xe2x80x9cProduction of Hydrogen and Sulphur from Hydrogen Sulfide in Refineries and Processing Plants,xe2x80x9d published by P. D. Clark and al. in the Sulphur 95 Conference, the impact on the final hydrogen yield, of a time of cooling to 500xc2x0 C., for a mixture of hydrogen, H2S and elementary sulfur at equilibrium at 1200xc2x0 C. is indicated. It is seen that to limit the hydrogen yield losses, a cooling time of less than 10 ms is necessary, and shorter periods on the order of 1 ms are even desirable. Such short cooling times are virtually impossible to achieve, however, by external heat exchange.
The approach is then to consider a cooling by mixing with a colder fluid (quench), but there again, it is technically difficult to attain mixing times that are short enough to avoid a significant recombining of hydrogen into H2S.
This invention has as its object a process and a device that make it possible to generate hydrogen from H2S by thermal method by eliminating drawbacks of the prior art, in particular by reducing the recombination of hydrogen into H2S with elementary sulfur formed during the cooling of the high-temperature gas and, by the use of mixers, that make it possible to obtain very short cooling times, generally less than 20 ms and preferably less than 2 ms, by mixing with a cold fluid so as to recover at least 80% of the hydrogen that is formed at equilibrium at temperature T1 and preferably at least 90%.
More specifically, the invention relates to a process for the production of a gas that contains hydrogen from a feedstock that contains at least H2S, comprising at least the following stages:
Stage 1: Generation of a gaseous effluent that contains at least the hydrogen and the elementary sulfur from the feedstock,
Stage 2: Separation of the effluent from the elementary sulfur and recovery of a gas that contains hydrogen,
characterized in that stage 1 comprises:
A stage for heating the feedstock that contains the H2S in at least one heating zone to a temperature T1 so as to produce a hot gas that contains at least hydrogen and elementary sulfur, obtained by decomposition of the H2S, whereby this heating is carried out at least in part by an external heat source,
A stage for quick cooling of the hot gas that is thus obtained, by mixing with a fluid FQ at a suitable temperature T2 by means of at least one mixer, preferably an ejector, so as to obtain the gaseous effluent at a suitable temperature T3, under conditions such that the mixing time is less than 20 milliseconds.
It is possible to use advantageously as a mixer a conventional liquid-gas ejector or a gas-gas ejector, preferably a gas-gas ejector that is suitable for mixing the gases well and whose motive fluid speed is advantageously supersonic.
Another object of the invention is to propose a method for heating the gas that contains H2S that makes it possible both to attain very high temperatures that promote the decomposition of H2S and the production of hydrogen, while avoiding recourse to an expensive external heat source such as electricity.
More particularly under this object, the invention relates to a process in which the stage for heating the gas that contains the H2S can be carried out in 2 consecutive sub-stages:
Heating of the gas that contains H2S to a temperature T4 by an external heat source,
Heating of the gas that is thus obtained of temperature T4 to temperature T1 by
reaction with a gas that contains oxygen, such as air, oxygen enriched air, or preferably industrially pure oxygen.
Finally, the invention has as its last object a process that combines the process of the invention with a hydrodesulfurization process, which makes it possible to minimize the equipment that is necessary for the purification and to use the hydrogen that is produced by the process"" of the invention.
According to this object, the invention relates to a process in which the gas mixture that is obtained from stage 2 and from which elementary sulfur and the major portion of the water are removed, can be compressed to be sent to a hydrodesulfurization unit that comprises a downstream amine scrubbing stage of the hydrogen containing effluent, and the resultant hydrogen can then be recycled to the hydrodesulfurization reaction section, upstream from said washing stage.
The invention also relates to a device for the implementation of the process. In particular, it relates to a unit for production of a gas that contains hydrogen, comprising at least one ceramic furnace, a supply of a feedstock that contains at least hydrogen sulfide, an output of an effluent that contains hydrogen, elementary sulfur and H2S, means for separating components of the effluent and means for collecting gas that contains hydrogen, connected to means for separation of the components of the effluent, wherein in said unit the furnace that is connected at least in part to an external heat source, and at least one means for mixing the effluent with a coolant, is connected to the outlet of the furnace, so as to cool the mixture that is thus obtained to a temperature T3, under conditions such that the mixing time is less than 20 milliseconds, and said mixing means is connected to the separating means.
According to a characteristic of the device, the furnace can comprise in a first part, on the feedstock feed side, means that are suitable for heating the feedstock with an external heat source, and in an adjacent second part, combustion means for combusting an effluent that is produced in the first part and which is connected to a means for input of a gas that contains oxygen, and said furnace has an outlet for passing the combustion effluent to mixing means.
According to a variant, the means for separating the components of the effluent can comprise at least one means (10) for condensation of the elementary sulfur that has an inlet connected to mixing means (7) and an effluent outlet that is connected to a heater (13), whereby said heater is connected to a reactor (15) for catalytic hydrogenation of the components of the effluent, whereby the reactor has an outlet connected to a water condenser (18), the water condenser has two outlets, one is connected via a water pumping means (25) and a heater (26) to an inlet of mixing means (7), the other outlet of the water condenser is connected to a unit (20) for separating the H2S/H2 components, whereby said separation unit is connected by one of its outlets that contains H2S to furnace (2) and by the other outlet to means (21) for collecting hydrogen.
A Claus unit that comprises a heater (33) that is connected to a Claus reactor (35) that contains a suitable catalyst that is connected in its turn to a sulfur condenser (38) can be interposed between sulfur condensation means (10) and heater (13) upstream from the catalytic hydrogenation reactor, whereby said condenser has a first sulfur outlet and a second outlet connected to heater (13) upstream from the catalytic hydrogenation reactor.